Transcript Types of Aqueous Reactions

Types of Aqueous
Reactions
1
Recognizing types of
Reactions
It is possible to predict the possible products of a
reaction based on the reactants.
Many of these predictions are easy to make if you
understand a few basic things about
molecules/atoms.
You, too, can learn to predict reaction products!
2
A Question
(NH4)2SO4 + FeCl3 →
What happens when you mix ammonium sulfate and
ferric chloride?
A better question is: what COULD happen?
3
(NH4)2SO4 + FeCl3 →
(NH4)2SO4
FeCl3
What kind of molecules are these?
Ionic!
In solution, what form do ionic compounds take?
Ions!
4
Are these the same?
(NH4)2SO4 + FeCl3 → Fe2(SO4)3 + NH4Cl
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(aq)
Kind of depends on what you mean by the same…
The “state of matter” is important. What if they were all solids?
5
Are these the same?
(NH4)2SO4 + FeCl3 → Fe2(SO4)3 + NH4Cl
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(aq)
(NH4)2SO4 (s) + FeCl3 (s) → Fe2(SO4)3 (s) + NH4Cl(s)
The bottom 2 are definitely different from each other. And only one of
them makes any sense!
6
(NH4)2SO4 + FeCl3 →
Fe2(SO4)3 + NH4Cl
(NH4)2SO4
FeCl3
In solution, what form do ionic compounds take?
NH4+ SO42-
Fe3+
Cl-
What about the products? Fe2(SO4)3
NH4Cl
In solution….
Fe3+ SO42- NH4+ ClNO DIFFERENCE! NO REACTION!
7
What are the possible
products?

(NH4)2SO4 (aq) + FeCl3 (aq) →

Fe2(SO4)3 + NH4Cl

Why?

Because + ions can only associate with negative
ions and – ions can only associate with positive ions
8
What are the possible
products?

(NH4)2SO4 (aq) + FeCl3 (aq) →
NH4+ SO42-
Fe3+
Cl-

Fe2(SO4)3 + NH4Cl


Are these products solids, liquids, gases, or aqueous?
Depends! They are ionic, so they could be solids. But, they could also be
aqueous. How?
If they are water soluble solids then they are aqueous! (The reactants are in
water already)
Does it make a difference?

You bet!!!!


9
Possible Reactions
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(aq)
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (s) + NH4Cl(s)
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(s)
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (s) + NH4Cl(aq)
The top one….?
No Reaction – just a bunch of ions in water!
10
Possible Reactions
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(aq)
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (s) + NH4Cl(s)
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(s)
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (s) + NH4Cl(aq)
These are all DIFFERENT reactions
How would you know which happens?
You need to know the solubility of the products.
11
Predicting Solubilities
Solubility is very complicated, but there are a few simple rules that help
with a large number of compounds:
1.
2.
3.
4.
5.
6.
Group 1A salts are all soluble.
All salts containing nitrates, ammonium, chlorate, perchlorate, and
acetate are soluble.
All Cl, Br, I salts are soluble EXCEPT for Ag, Pb, and Hg22+ salts
All sulfates are soluble EXCEPT Pb, Ca, Ag, Sr, Hg22+ and Ba
Metal hydroxides are INSOLUBLE except for those of Group 1A and
Ca, Sr, and Ba
All salts containg phosphate, carbonate, sulfite, and sulfide are
insoluble EXCEPT for those of Group 1A and NH4+
12
13
Based on those rules…
(NH4)2SO4 (aq) + FeCl3 (aq) → Fe2(SO4)3 (aq) + NH4Cl(aq)
(NH4)2SO4 (aq) + FeCl3 (aq) → No Reaction
14
Precipitation Reactions
This is an example of a “double-replacement reaction”
– swapping cations between anions.
This is also an example of a “precipitation reaction” –
forming a solid from a reaction performed in
aqueous solution. (Although no precipitation
reaction occurred in this instance due to solubility of
the products.)
15
Another Example
Fe2(SO4)3 (aq) + Ca(NO3)2 (aq) → ???
What’s the first thing to consider?
What is the nature of the reactants?
They are ionic!
16
Another Example
Fe2(SO4)3 (aq) + Ca(NO3)2 (aq) → ???
What’s the first thing to consider?
What is the nature of the reactants?
They are ionic!
Fe3+(aq) + SO42-(aq) + Ca2+(aq) + NO3-(aq) → ???
17
Possible Reactions
Fe3+(aq) + SO42-(aq) + Ca2+(aq) + NO3-(aq) → ???
Double Replacement!
Fe3+(aq) + SO42-(aq) + Ca2+(aq) + NO3-(aq) → Fe(NO3)3 + CaSO4
Next thing to consider…?
Soluble or insoluble?
18
Possible Products
Fe3+(aq) + SO42-(aq) + Ca2+(aq) + NO3-(aq) → Fe(NO3)3 + CaSO4
Fe(NO3)3???
Soluble! (All salts containing nitrates, ammonium, chlorate, perchlorate,
and acetate are soluble.)
CaSO4???
Insoluble! (All sulfates are soluble EXCEPT Pb, Ca, Sr, Hg22+ and Ba)
19
Actual Reaction…
Fe2(SO4)3 (aq) + Ca(NO3)2 (aq) →
Fe(NO3)3 (aq) + CaSO4 (s)

This is a precipitation reaction and a doublereplacement reaction!
Fe2(SO4)3 (aq) + 3 Ca(NO3)2 (aq) →
2 Fe(NO3)3 (aq) + 3 CaSO4 (s)
20
Net Ionic Equation
Sometimes, to simplify the expression down to
its most important elements, rather than write
the full chemical equation it is distilled down
to a net ionic equation
We ignore spectator ions (dissolved ions that
don’t change in the reaction) and write only
the species that change.
21
Writing the Net Ionic Equation
Fe2(SO4)3 (aq) + 3 Ca(NO3)2 (aq) →
2 Fe(NO3)3 (aq) + 3 CaSO4 (s)
Rewrite with aqueous ionic species as ions:
2 Fe3+(aq) + 3 SO42-(aq) + 3 Ca2+(aq) + 6 NO3-(aq) → 2 Fe3+(aq) + 6 NO3-(aq) +
3 CaSO4 (s)
Cancel things that appear on both sides!
3 SO42-(aq) + 3 Ca2+(aq) → 3 CaSO4 (s)
SO42-(aq) + Ca2+(aq) → CaSO4 (s)
22
3 SO42-(aq) + 3 Ca2+(aq) → 3 CaSO4(s)
Advantages of the Net Ionic Equation:
1.
2.
3.
It is complete – all changes are spelled out.
It is concise – only things that actually change in
the reaction are shown.
It is the simplified recipe – if I want to make
CaSO4, does it matter what I use as the source of
SO42-?
23
Precipitation Reactions
It should be possible for you to recognize the
possibility of a precipitation reaction:
1. Recognize the reactants are ionic
2. The ionic species are aqueous
3. Double replacement products are,
therefore, possible
4. One or more of the possible products is
insoluble.
24
Clicker Question #2
Write the correct balanced equation for the following
reaction:
K2SO4 (aq) + Pb(NO3)2 (aq) 
A. K2SO4 (aq) + 2 Pb(NO3)2 (aq)  PbSO4 (aq) + 2 KNO3 (aq)
B. K+(aq) + NO3-(aq)  KNO3 (s)
C. K2SO4 (aq) + 2 Pb(NO3)2 (aq)  PbSO4 (s) + 2 KNO3 (s)
D. Pb2+(aq) + SO42-(aq)  PbSO4 (s)
E. No Reaction
25
Predicting Solubilities
Solubility is very complicated, but there are a few simple rules that help
with a large number of compounds:
1.
2.
3.
4.
5.
6.
Group 1A salts are all soluble.
All salts containing nitrates, ammonium, chlorate, perchlorate, and
acetate are soluble.
All Cl, Br, I salts are soluble EXCEPT for Ag, Pb, and Hg22+ salts
All sulfates are soluble EXCEPT Pb, Ca, Sr, Hg22+ and Ba
Metal hydroxides are INSOLUBLE except for those of Group 1A and
Ca, Sr, and Ba
All salts containg phosphate, carbonate, sulfite, and sulfide are
insoluble EXCEPT for those of Group 1A and NH4+
26
Acid-base reactions
Another type of aqueous chemical reaction is an
acid/base reaction – the reaction between an acid
and a base.
There are different types of acids and bases. We rely
on the Bronsted-Lowry definition:

Bronsted-Lowry acid – proton (H+) donor

Bronsted-Lowry base - proton (H+) acceptor
27
Recognizing acids and bases
Acids are easiest to recognize in this system.

Bronsted-Lowry acid - proton donor
If you are going to donate a proton, what must be
true?
You must have a proton!!!
28
Which of these compounds is an
acid?
HCl
Fe2(SO4)3
NaOH
NaNO3
H2SO4
29
Which of these compounds is an
acid?
HCl – yep, it has a proton
Fe2(SO4)3 – nope, it has no proton
NaOH – nope, it has no proton! (This is the trick question,
hydroxide is a unit, there isn’t a separate proton there)
NaNO3 - nope, it has no proton
H2SO4 – yep, it has a proton – two, in fact.
30
Recognizing acids and bases
Bases are a bit harder to recognize in this system.

Bronsted-Lowry base - proton acceptor
If you are going to accept a proton, what must be true?
You must have available electrons – H+ is a cation, it
needs electrons.
31
Strong vs. weak
Electrolytes (soluble ionic species) in general, and acids and bases
in particular, are considered to come in two types: strong and
weak.
This is not what it sounds like! It has nothing to do with how
powerful the solutions is.
A strong electrolyte is one that completely dissociates into its ions
in water. A weak electrolyte is one that only partially
dissociates into its ions in water.
32
Aqueous Acids & Bases
The key to the aqueous chemistry of acids and bases
is WATER! (Huge surprise, I know! )
Water, among its many interesting properties, is also
amphiprotic. Water is both an acid and a base!
H2O – It has a proton and it has excess electrons on
the oxygen.
33
General Acid/Base Reaction
H-A + X-B → H-B + X-A
(where X and A are any chemical species, H is
the proton, and B is any basic species)
34
Products of Acid/Base
reactions
All acid/base reactions have the same two net
products: water & a salt!
HCl + NaOH → H2O + NaCl
H2SO4 + LiOH → H2O + LiHSO4
HCl + NH3 → Cl- + NH4+ Where’s the water?
35
The role of water
NH3 is a weak base – ammonia.
Any weak base in water will accept a proton from
water.
NH3 (aq) + H2O(l) → NH4+(aq) + OH- (aq)
NH3 (aq) + H2O(l) → NH4OH(aq)
Ammonia and ammonium hydroxide are used
interchangeable.
36
The role of water
HCl + NH3 → Cl- + NH4+
NH3 (aq) + H2O(l) → NH4OH(aq)
HCl + NH4OH → H2O + NH4Cl
This is the reason for the old Earth Science myth that
all bases have OHThey DO, as long as they are in water!
37
Gas Evolution Reactions
Just what it sounds like: reactions that create a gas as a product.
These reactions can be difficult to identify. A couple guidelines:
1.
Sulfides tend to create gas products: H2S(g)
2.
Carbonates and bicarbonates (CO32- and HCO3-) form
compounds that break down into gases: H2CO3 (aq) breaks
down into H2O (l) and CO2(g).
3.
Sulfites and bisulfites (SO32- and HSO3-) form compounds that
break down into gases: H2SO3 breaks down into SO2(g)
4.
Ammonium compounds (NH4+) can form compounds that break
down into NH3(g): NH4OH(aq) breaks down into H2O(l) and
NH3(g).
38
Gas Evolution Reaction Examples
2 HCl(aq) + K2S(aq) 
The key here is to see the sulfide. In solution, both of
these compounds are ionic and break down into
ions:
2 H+ (aq) + 2 Cl- (aq) + 2 K+(aq) + S2-(aq) 
So, it is still a double replacement reaction:
2H+ (aq) + 2Cl- (aq) + 2 K+(aq) + S2-(aq)  H2S + 2KCl
39
Gas Evolution Reaction Examples
2 HCl(aq) + K2S(aq)  H2S + 2KCl
But, like with the precipitation reactions, we need to determine what
kind of compound the products are.
KCl?
Ionic, so it dissociates!
H2S?
Actually, it is Jumping Jack Flash!
It’s a Gas! Gas! Gas!
40
Gas Evolution Reaction Examples
2 HCl(aq) + K2S(aq)  H2S (g) + 2KCl(aq)
Or, if you want to write it as a net ionic equation:
2H+ (aq) + 2Cl- (aq) + 2 K+(aq) + S2-(aq)  H2S(g) +
2K+(aq) + 2Cl-(aq)
Cancel the “spectators”
2H+ (aq) + S2-(aq)  H2S(g)
41
Gas Evolution Reaction Examples
2H+ (aq) + S2-(aq)  H2S(g)
This really shows you a whole class of gas
evolution reactions. Take any acid (source of
H+) and any soluble sulfide (source of S2-)
and you get H2S (g)!
42
So, in 10 seconds or less!
HCl(aq) + Li2S(aq) 
H2S(g) + LiCl(aq)
H2SO4(aq) + FeS(aq) 
H2S(g) + FeSO4(aq)
43
How to make a bomb!
http://www.youtube.com/watch?v=0fdbzEpm4D
I&feature=related
44
http://www.youtube.com/watch?v=4N0m95PEx
HY&feature=related
45
Another tricky type of
reaction:
Oxidation-reduction reactions (“redox
reactions”) are barely chemical reactions at
all.
What is a chemical reaction?
A process in which bonds are broken and made
so that atoms change partners.
46
Consider 2 molecules
FeO and Fe2O3
Are they different?
Yes.
What’s the difference?
Iron (II) oxide vs. Iron (III) oxide The Oxidation State is
different.
47
What is an “oxidation state”?
The simplest way to think about an “oxidation
state” is that it is the charge the atom has or
could have if you separated it from the atoms
it is bonded to.
48
Are you stuck with your oxidation
state?
Asked a different way: If you are iron in FeO,
are you stuck being Fe2+ forever?
In fact, you can change oxidation states as
often is you like. But, there’s a catch…
How do you change oxidation states?
Add or subtract electrons. Fe2+ has 1 more
electron than Fe3+
49
What does this reaction look
like?
Fe2+  Fe3+ + 1 eIs this a “real” reaction?
Depends on what you mean by “real” and by
reaction. Something changed, but no atoms
were rearranged so it isn’t like the other
reactions we’ve seen before. And, you might
ask, what happens to the electron?
50
This is an “electrochemical”
reaction
Fe2+  Fe3+ + 1 eIt’s a special kind of process, part electrical and
part (barely) chemical. The atom changes
oxidation state and creates an electron. The
electron can do useful work (power your
Ipod) or chemical work (change the oxidation
state of something else).
51
Electrons come, electrons go
Fe2+  Fe3+ + 1 eMn5+ + 3 e-  Mn2+
When electrons “go”, it is called an “oxidation”.
When electrons “come”, it is called a
“reduction”.
[It’s easiest to remember that a “reduction”
reduces the charge on the ion (oxidation
state).]
52
Like acids and bases…
Oxidation and Reduction always happens
simultaneously:
Oxidation half-reaction: Fe2+  Fe3+ + 1 e-
Reduction half-reaction: Mn5+ + 3 e-  Mn2+
Full reaction: 3 Fe2+ + Mn5+  3 Fe3+ + Mn2+
WTFDYGT?????????????
53
Chemical reactions don’t have
electrons
Oxidation and Reduction half-reactions balance so that
no NET electrons remain
Oxidation gives you 1 e-: Fe2+  Fe3+ + 1 eReduction needs 3: Mn5+ + 3 e-  Mn2+
3 x (Fe2+  Fe3+ + 1 e-)
+ Mn5+ + 3 e-  Mn2+
3 Fe2+ + Mn5++ 3e-  3 Fe3+ + Mn2+ + 3e3 Fe2+ + Mn5+  3 Fe3+ + Mn2+
54
Is it always that easy?
Of course NOT!
We’ll talk about these reactions in much greater
detail later. For now, we just want to
recognize one when we see it. We’ll figure
out how to balance them after the snow
melts.
55
Determining oxidation states
1.
2.
3.
4.
5.
6.
Free elements are always 0 (Cu, Fe, H2)
The oxidation state of a monoatomic ion is just its
charge (Cu2+, Fe3+, S2-)
The sum of the oxidation of all atoms in a
compound or ion is the charge of the compound or
ion (H2O, NH4+)
Metals are always positive. Group 1A metals are
ALWAYS +1. Group 2A metals are ALWAYS +2.
(NaCl, Ca(OH)2)
Halogens (Group VIIA) are 95% of the time -1.
Chalcogenides (Group VIA) are 95% of the time 2.
56
Ionic compounds
For an ion or an ionic compound, the oxidation
state is easy: it’s the charge on the ion.
FeCl3 – Fe must be +3 because there are 3 Clions stuck to it.
MnO2 – Mn must be +4 because there are 2 O2stuck to it.
57
Covalent compounds
Here is where it is a little tricky. The atoms in a
covalent compound don’t have a real charge on
them. BUT, they do have a potential charge if you
pulled them all apart and the electrons they share
get split up.
CO – carbon must be +2 since O wants to be -2
CO2 – carbon must be +4 since there are 2 O that
want to be -2 each
58
59
What’s the oxidation state of
the atoms?
SrBr2
Br is a halogen – it must be -1 when bonded to
a metal
Sr must be +2
SO3
O is usually -2, which means S must be +6
60
What’s the oxidation state of
the atoms?
CO32O is usually -2, which means C must be +4
since the entire molecule is -2
NO3O is usually -2, which means N must be +5
since the entire moecule is -1
61
KI
What’s the oxidation state of K?
A.
B.
C.
D.
E.
+1
-1
+2
-2
Cannot be determined.
62
KI
K is a Group IA metal. It is +1 ALWAYS!
I is, therefore, -1 (as it usually is for halogens)
because the whole molecule must add up to
ZERO.
63
MnO
What is the oxidation state of Mn?
A.
B.
C.
D.
E.
+2
+4
+1
-1
-2
64
MnO
Mn is not in one of our 1st two columns. So we
can’t know its oxidation state directly.
HOWEVER, oxygen is Group VIA and is almost
always -2.
So, if O is -2, Mn must be +2 for the whole
molecule to be neutral.
65
2 CuO + 2 FeO  Fe2O3 + Cu2O
Redox reaction? You bet!
How can you tell? Two things are changing
oxidation state. (There must always be an
oxidation and a reduction.)
What are the oxidation states?
66
2 CuO + 2 FeO  Fe2O3 + Cu2O
CuO - Copper is
+2
How do you know?
Because O is -2
Cu2O – Copper is…
+1
How do you know?
Because O is -2
67
As soon as you see one thing
changing oxidation state…
…there is another! (Yoda 1980)
There has to be an oxidation and a reduction.
2 CuO + 2 FeO  Fe2O3 + Cu2O
FeO – Fe is +2 (because O is -2)
Fe2O3 – Fe is +3 (because O is -2)
68
2 CuO + 2 FeO  Fe2O3 + Cu2O
Cu goes from +2 (CuO) to +1 (Cu2O) while Fe
goes from +2 (FeO) to +3 (Fe2O3)
So Fe is giving up an electron (going from +2 to
+3) while Cu is gaining an electron (going
from +2 to +1).
69
Clicker Question
Which of the following is a redox reaction:
A. Li(s) + O2 (g)  Li2O (s)
B. Pb(NO3)2 (s) + Na2SO4 (s)  PbSO4 (s) + 2 NaNO3 (s)
C. Mg(s) + Br2 (l)  MgBr2 (s)
D. A and C
E. A, B, and C.
70